As being self-luminescent, EL devices have high visibility. In addition, as being completely solid, EL devices have higher impact resistance than liquid crystal devices. Therefore, the use of EL devices in various displays as light emitters is being widely noticed. EL devices are grouped into inorganic EL devices and organic EL devices, the former containing inorganic compounds in their light-emitting layers while the latter containing organic compounds. For organic EL devices containing organic compounds in their light-emitting layers, various structures of various materials have heretofore been proposed capable of giving high luminance at low voltage of direct currents.
Vacuum vapor deposition is one typical method of producing organic EL devices, which comprises vaporizing predetermined organic compounds under heat to form films on substrates. However, conventional organic EL devices, even though having good initial capabilities, are often problematic in that their capabilities are greatly reduced within a short period of time. It has heretofore been difficult to produce organic EL devices all the time stably having the designed capabilities. It is known that contamination of substrates causes the reduction in the capabilities of EL devices. In particular, in organic EL devices, contamination of the electrode/organic layer interface and the organic layer/organic layer interface is one factor of interfering with the carrier injection from the substrate electrode into the organic layers, thereby having noticeable negative influences on the capabilities of the devices.
In order to prevent the reduction in the capabilities of EL devices by such contamination, disclosed is a method of cleaning the surface of an electrode-mounted substrate to make it have a contact angle with water of smaller than 25.degree., prior to forming organic layers on the substrate (see Japanese Patent Application Laid-Open (JP-A) No. 220873/1995). In this method, the cleaned substrate is stored in vacuum or in inert gas to prevent it from being contaminated.
Also disclosed is a method of continuously producing organic EL devices all the time in vacuum, in which is used a vacuum chamber having therein a plurality of vacuum processing rooms, and each one of plural layers is formed in each room successively one after another (see JP-A No. 111285/1996). These vacuum rooms each have therein a source material to be vaporized for forming the intended film. A substrate is disposed in a predetermined one of those rooms, then the gate valve is shut, and thereafter the deposition source in the room is heated to form a film on the substrate through vapor deposition from the source.
However, organic EL devices as produced according to the method disclosed in JP-A No. 220873/1995 could not still have stable capabilities.
The method disclosed in JP-A No. 111285/1996, in which the deposition source is heated after the substrate has been disposed in the vacuum chamber, is problematic in that impurities having vaporized in the initial stage of heating deposit on the substrate to thereby worsen the capabilities of the devices produced. This is because the deposition sources, such as organic compounds for constituting organic layers and metals for constituting electrodes, are exposed in air prior to being set on the holders, and therefore impurities in air, such as organic substances, carbon dioxide, water, oxygen and the like adhere onto them. In particular, since synthetic organic compounds are formed in solvent, the solvent used often remains in the compounds. As a result, the deposition source of such a synthetic organic compound shall contain the solvent as an impurity. The impurity vaporizes in the initial stage of heating, thereby causing contamination of the substrate and the film formed thereon.